For a pathogen with R0 = 3.0, what herd immunity threshold (proportion immune) is required to prevent sustained transmission in a fully susceptible population (assume perfect vaccine efficacy)?

• A. About 90% immune is required for any pathogen regardless of R0, since herd immunity thresholds are effectively the same across viruses and bacteria in real populations.
• B. Approximately 33.3% immune is sufficient because only the inverse of R0 matters for herd protection and the threshold is 1/R0 rather than 1 - 1/R0.
• C. Approximately 66.7% of the population must be immune to prevent sustained transmission when R0 = 3.0 and vaccine efficacy is perfect.
• D. Herd immunity cannot be achieved with an R0 of 3.0, so no finite percentage of immune individuals will prevent sustained transmission in a population.

Answer: (C)